Solve for $p$ : $13 = 15 + p$
Subtract $15$ from both sides: $ \begin{eqnarray} \\ \hphantom{13 {- 15}} &\hphantom{=}& \hphantom{p} \\ 13 &=& 15 + p \\ \\ {-15} && {-15} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 13 {- 15} &=& p \\ \end{eqnarray} $ Simplify. $ p = -2$